1. Field of the Invention
The present invention relates to a toner and a production process for the same, the toner used for developing an electrostatic image formed on a photosensitive member provided in an image forming apparatus such as copiers and printers. More particularly, the invention relates to a toner which includes core particles containing at least a resin and a colorant and coated with a coating layer, and to a process for producing the toner.
2. Description of the Related Art
The image forming apparatuses such as copiers and printers have conventionally used a toner for developing an electrostatic image formed on the photosensitive member.
A milling process is widely used for producing such a toner. The process includes the steps of: admixing additives including a colorant, a wax and the like to a resin; heat melting the mixture and kneading the molten mixture; cooling the kneaded product; and milling the product into toner particles of a predetermined particle size.
Recently, for power saving purpose and such, the above image forming apparatuses have adopted the practice of fixing a toner image formed on a recording medium at lower temperatures and adequately fixing the toner image to the recording medium without using an oil. The practice dictates the need to admix an increased amount of wax to the aforesaid toner.
Furthermore, the aforesaid toner also need be admixed with an increased amount of colorant in order to ensure the formation of images having a sufficient image density.
In a case where the toner is produced by the aforementioned milling process, however, the following problem exists. If the toner is admixed with greater amounts of wax and colorant, the toner suffers increased amounts of wax and colorant exposed to toner surface. This results in serious degradation of the chargeability and the like of the toner.
More recently, a granulation process in a wet system, such as a suspension polymerization process, an emulsion dispersion process and an emulsion polymerization/flocculation process, has attracted attention as a toner production process.
Unfortunately, in a case where the toner is produced by such a granulation process in the wet system, it is impossible to put defective products to reuse for toner production and hence, the production cost is increased. Furthermore, the toner production using this method also involves the problem. That is, if the wax and colorant are added in greater amounts, the toner suffers the increased amounts of wax and colorant exposed to the surface thereof. Hence, the toner is seriously degraded in the chargeability and the like.
More recently, there has been proposed a method which uses a salting-out/fusion-bonding process or the like for fusion-bonding resin microparticles to the surface of core particles in a wet system thereby forming a coating layer over the core particles containing a resin, a colorant and the like (see, for example, U.S. Patent Application Publication US 2002/0039699 and Japanese Unexamined Patent Publication No. 2002-229251).
However, where the salting-out/fusion-bonding process is used for fusion-bonding the resin microparticles to the core particle surface in the wet system thereby forming the coating layer thereover, various problems are encountered. That is, water, dispersant and the like remain in the toner so that the toner is reduced in the chargeability. In addition, the aforesaid coating layer suppresses the functions of the wax and colorant so that the toner is reduced in fixing performance or cannot provide a sufficient image density.